When Cassini’s journey ends in 2017, it will have completed 293 orbits of Saturn and corrected its trajectory hundreds of times. Enormous planning goes into every Cassini observation, but the element of the mission that makes it possible to visit all the wonders of the Saturn system is navigation.
Cassini has unraveled mysteries galore in the Saturn system by being where it was supposed to be, when it was supposed to be there. Yet Cassini arrived with but a fraction of the propellant needed to visit all the places on its itinerary. Instead of propellant, the navigation team uses Saturn’s largest moon Titan to make the biggest changes to the spacecraft’s trajectory.
Titan Does the Heavy Lifting
Cassini’s combined orbits around Saturn look like a ball of yarn exploded without the threads breaking. The loops reach in all directions and are long in places but short in others. Though they appear disorderly, the sum of Cassini’s orbits represents a carefully choreographed, decade-long dance between Cassini and Titan.
“Titan is the engine of this tour,” said Duane Roth, chief of Cassini’s navigation team. Since Cassini arrived at Saturn in 2004, Roth and his team of astrodynamicists at NASA's Jet Propulsion Laboratory have used Titan to send the Cassini spacecraft swinging up, down and around the Saturn system.
Cassini’s total propellant at launch was enough to alter the spacecraft’s velocity by only about 5,400 miles per hour (2,400 meters per second), half of which was gone by the time the spacecraft reached Saturn. However, a single flyby of Titan at an altitude of 620 miles (about 1,000 kilometers) gives Cassini a change in velocity of about 1,800 miles per hour (800 meters per second) — this is equivalent to one-third of Cassini’s total propellant at launch.
A single flyby of Titan at an altitude of 620 miles (about 1,000 kilometers) gives Cassini a change in velocity of about 1,800 miles per hour (800 meters per second) — this is equivalent to one-third of Cassini’s total propellant at launch.
In spaceflight, change in velocity is called “delta-v.” The word “delta” refers to the Greek letter “D” and is used as a symbol in math and science formulas to mean “change,” and “v” refers to “velocity.” So “delta-v” or ∆ literally means “change of velocity.”
By mission’s end, Cassini will have achieved a delta-v of about 200,000 miles per hour (about 90,000 meters per second) from Titan flybys — roughly 37 times what it could ever have achieved by propellant alone. “What we carry in the spacecraft is a pittance compared to what we get from Titan,” Roth said.
When Cassini passes relatively close to Titan, the moon’s gravitational influence grabs the spacecraft tightly and swings it around in a sharp turn. If Cassini performs a higher-altitude flyby, Titan’s grip is looser and the spacecraft’s trajectory is altered less. If Cassini passes by Titan’s southern hemisphere, the spacecraft’s path carries it “up” above Saturn’s ring plane (northward, relative to Saturn's equator), and if Cassini passes over Titan’s north, the spacecraft’s path bends southward, relative to the planet. In that way, Titan serves as a pivot point for Cassini.
Generally, Cassini uses propellant only to make small corrections that nudge it back toward its intended and ideal trajectory (called the “reference trajectory”) for the next Titan flyby. And each Titan flyby is designed to give Cassini the right direction and speed for its next Saturn orbit, lasting from a week to several months, during which it will observe particular moons or Saturn’s rings, for example. The encounters also line up the spacecraft for its next Titan flyby.
“Every Titan flyby is a setup for the rest of the Titan flybys,” Roth said. If Cassini is off target in a flyby by half a mile (a kilometer), the spacecraft can burn a little propellant to correct the error. But if it misses by tens of miles, scientific observations might have to be rescheduled or even canceled. “It would take about six months to re-synchronize the trajectory with the science observations and would be like losing six months from the mission,” Roth said. Fortunately, that sort of replanning has not been necessary since Cassini arrived at Saturn.
But, surprisingly, even though Titan has played this essential role in Cassini's mission, no one knows where the moon is with absolute certainty.
Locating Titan, And Cassini
Prior to Cassini’s arrival, Titan was observed primarily from Earth and Earth orbit, where it appears as a tiny point of light. Voyagers 1 and 2 imaged Titan, but their observations were limited in time. The Voyager spacecraft raced through the Saturn system – they didn’t orbit Saturn.
“To complicate things, Titan has an atmosphere, so it looks like this fuzzy ball,” Roth said. “We missed our first flyby of Titan by a distance on the order of 20 kilometers [12 miles].” Since then, Cassini’s navigation team has used each flyby to improve the accuracy of their knowledge of the moon’s orbit.
Perhaps more surprising, the exact position of Cassini itself is something of a permanent mystery. The navigation team estimates Cassini’s location by sending radio signals to the spacecraft from the powerful antennas of NASA's Deep Space Network. When Cassini receives the signal more than an hour later, it immediately sends it back to Earth. Because the signal travels at a fixed speed — the speed of light — the team can calculate Cassini’s distance by knowing precisely when the signal was sent from Earth and received from the spacecraft. And by measuring the shift in the frequency Cassini sends back (due to the Doppler effect), the navigation team calculates Cassini’s speed toward or away from Earth.
Like mariners of old, Cassini navigators also use the stars to understand their ship's position. The spacecraft’s cameras collect "optical navigation" images of Saturn's moons against a background of stars whose positions are well-known from astronomical measurements. But Cassini’s position is never nailed down with absolute certainty. “We can never know exactly where the spacecraft is,” Roth said. "But we've managed to do some amazing things flying the spacecraft with the knowledge we have.”
Among those amazing things was collaborating with ESA (the European Space Agency) to overcome a critical challenge early in the mission.
Science through Teamwork
Having traveled together for seven years, the Cassini spacecraft and ESA’s Huygens probe would part ways not long after arriving at Saturn. As the first human-made probe to land on a moon in the outer solar system, Huygens was to parachute onto frigid Titan and send data to Cassini about the moon’s temperature, pressure and chemistry, and even photos from the surface.
But before Cassini-Huygens reached Saturn, ESA engineers determined that the Doppler shift of the probe’s data transmission was not properly accounted for in the design of the receiver system. Cassini was supposed to follow a path directly behind Huygens where the Doppler shift would be large. That meant Huygens would be sending its data at a frequency that its receiver aboard Cassini couldn’t translate.
The European Space Agency noticed something wrong and they gave us time to fix it. They deserve a lot of credit for identifying the problem in flight.
Working together with their European colleagues, Cassini’s navigation team helped find a solution. “We changed the trajectory of Cassini so that its position was nearly perpendicular to the probe’s path,” Roth said. There the Doppler effect was minimal and the Huygens mission was able to proceed to great success.
“The European Space Agency noticed something wrong and they gave us time to fix it,” Roth said. “They deserve a lot of credit for identifying the problem in flight.”